The art of lithographic printing is based on the immiscibility of oil and water. Lithographic printing plates have ink receptive areas generated on a hydrophilic surface. When the surface is moistened with water and ink is applied, the hydrophilic surface retains the water and repels the ink, and the ink receptive areas accept the ink and repel the water. The ink may then be transferred to the surface of a material upon which the image is to be reproduced. Typically, the ink is first transferred to an intermediate blanket, which in turn transfers the ink to the surface of the material upon which the image is to be reproduced.
Lithographic printing plates typically comprise a radiation-sensitive coating applied over the hydrophilic surface of a substrate. After a portion of the coating is exposed to radiation (commonly referred to as image-wise exposure), the exposed portion becomes either more soluble or insoluble in a developer than an unexposed portion of the coating. A printing plate is generally considered a positive-working plate if, after exposure to radiation, the exposed regions of the radiation-sensitive coating become more soluble and are removed in the developing process revealing the hydrophilic surface. Conversely, if the exposed regions become more insoluble in the developer and the unexposed regions are removed in the developing process, the plate is considered a negative-working plate. In each instance, the undeveloped areas provide an ink-receptive image, while the regions of the hydrophilic surface exposed during development repel ink.
Traditional positive-working printing plates are sensitive to ultraviolet radiation and have excellent developer resistance prior to image-wise exposure. Thermally sensitive (i.e. IR sensitive) positive-working printing plates have also been developed. One drawback to these thermally sensitive printing plates are that they have just-adequate developer resistance prior to imaging. One method of improving developer resistance in thermally imageable systems is to add solubility inhibitors to the radiation-sensitive layer. The solubility inhibiting component may be a separate compound or be part of the material composing the radiation-sensitive layer. After image-wise thermal exposure of the radiation-sensitive layer (including a solubility inhibitor), the rate of dissolution of the exposed areas in a developer is substantially greater than the rate of dissolution of the unexposed regions. In this manner, the exposed regions may be removed by the developer, while the unexposed regions produce a developer-resistant image.
Materials including quinonediazides are suitable solubility inhibitors. Materials including quinonediazides provide improved plate resolution, aging characteristics and developer resistance, while maintaining a desired imaging speed. However, one drawback to printing plates incorporating quinonediazides is that the plate suffers from increased white light sensitivity, causing the quinonediazides to decompose when subjected to white light. This produces an extremely developer soluble plate in which portions of the plate that are not thermally image-wise exposed may be undesirably developed.